1. Field of the Invention
This invention relates to measuring and testing corrosion processes, and relates particularly to the instruments and electrochemical techniques used in the study of corrosion processes.
2. Description of the Prior Art
The determination of corrosion attack within pipelines carrying corrosive liquids or gases at high pressures is becoming increasingly important for operating safety and monitoring the selective addition of corrosion inhibitors. The transmission of combustible fluids from source to market is provided by pipelines which may vary in dimension from a few inches in diameter to as large as several feet in diameter. Many of these pipelines operate at elevated pressures as high as 1400 psi. There are approximately a million miles of pipelines within the United States which could be damaged from undetected corrosion. In many of these pipelines, a sufficient amount of a liquid aqueous phase is present that a polarization-type corrosion detecting probe and instrument may be employed. It has been found that even in high-pressure gas lines, a very thin film of water coats the interior surface of the pipeline. The placement of the corrosion sensing electrodes of the corrosion prove within such liquid film is essential for proper corrosion measurements.
A probe assembly has been developed for the polarization determination of corrosion effects within a pipeline. This new probe assembly provides for precise placement of planar surfaced electrodes in parallel alignment with the inner surface of the pipeline and within the thin liquid film carried therein. This unique flush-mounted probe assembly is described in U.S. Pat. No. 3,910,830. In a preferred embodiment of the probe assembly, a novel probe is removably mounted within an internally shouldered nipple welded to the pipeline by "hot tapping" techniques. The probe has a cylindrical body with an annular seal adjacent to a sensing head which carries the flat-surfaced metal electrodes. An adjustment nut threadily mounts on a reduced diameter rear portion of the body and engages the shoulder on the nipple to precisely position the electrodes in parallel alignment with the inner wall surface of the pipeline. The nut is adjusted and locked on the body after measurements of the relative longitudinal dimensions from the shoulder to the interior wall surface of the pipeline. This unique flush-mounted probe assembly has found acceptance in corrosion measurements of pipeline but requires careful hot-tapping techniques for installation and removal.
Installations of flush-mounted probe assemblies are usually made in the field by non-technical personnel who are accustomed to installing valves, coupon holders, blind plugs and like fittings with conventional hot-tapping machines. The flush-mounted probe of the present invention has appearance, construction, and function to be completely compatible with the normal nipples and fittings employed in hot-tapping operations in pipelines. Additionally, the present flush-mounted probe does not require technical personnel, such as corrosion engineers, for proper installations and usuage.
It is preferred for rapid and accurate results with this novel flush mounted probe to measure the corrosion occuring within a pipeline by employing the electro-chemical process and apparatus generically described in U.S. Pat. No. 3,406,101. In this technique, there is employed a corrosion ratemeter which connects with a probe having three metal electrodes adapted to be exposed to a corrosive liquid. The instrumentation includes an adjustable current source, an ammeter, and a high impedance volt meter as the primary components. The adjustable current source applies a small electric current between a "test" electrode and an auxiliary "electrode." At the same time, the volt-meter monitors the induced polarization potential between the test electrode and a "reference" electrode. The current flow slightly polarizes the surface of the test electrode, and as a result, causes a shift in the potential between the test and reference electrodes. The current flow required to produce about 10 millivolts polarization is directly proportional to the corrosion rate of the test electrode undergoing corrosion.
In many pipelines the effects of corrosion are very long term and should be monitored on a continuous basis over a selected period of time. One corrosion ratemeter for industrial monitoring of corrosion effects for extended periods of time is shown in U.S. Pat. No. 3,717,566. This corrosion ratemeter is automatic in operation between nulling and recording with a permanent chart record of highly accurate measurements of corrosion over extended periods of time. If desired, other corrosion ratemeters of similar manufacture for making corrosion measurements may be employed to advantage.
It is especially advantageous to employ these automatic recording corrosion ratemeters with the novel probe assembly of this invention for monitoring directly the corrosion occurring in pipelines. The flush mounted probe of the present invention provides simple, easy, and full-proof introduction of sensing electrodes into the pipeline to be monitored in such a manner that the flat-surfaced electrodes are precisely aligned with the inner sidewall surface of the pipeline. Normal operation of the pipeline is not disturbed. The placement of the flush mounted probe is completely conventional by hot-tapping techniques within the pipeline. Other results provided by the present flush mounted probe will be appreciated from the following discussion.